An electrical energy unit referred to in the present invention is usually rechargeable and has a direct-current (DC) voltage. And an electrical energy unit may further comprise one or more sub-units; and the sub-units may be connected in series or in parallel or in any combination thereof to form the electrical energy unit. For instance, an electrical energy unit can be one battery cell, or it can be a battery module comprising a plurality of battery cells which are connected in series or in parallel or in any combination thereof to form the battery module.
Re-charging a string of series-connected electrical energy units involves adding charge to the entire string; while balancing the string involves redistributing charge among some electrical energy units within the string, but not adding any external charge to the string. As a technical terminology, “charge balancing” is sometimes interchangeably referred to as “charge equalization” or “charge redistribution” or simply “balancing”. A good example is balancing a lithium-ion battery string/pack for an electric car or a hybrid car, because mismatches in voltages, state-of-charge (SOC), capacities, internal impedances, and so forth among battery cells tend to increase over usage, over temperature, and over time. Battery balancing is one of the key functions of a battery management system (BMS). And a battery balancer is a dedicated device that can perform the task of battery balancing.
There are two basic categories of balancing technology, i.e., dissipative balancing and non-dissipative balancing. Dissipative balancing is sometimes referred to as passive balancing. Dissipative balancing cannot transfer charge among electrical energy units, but dissipates and therefore wastes excessive charge as undesirable heat usually when a string is being re-charged. Non-dissipative balancing is sometimes referred to as active balancing. Since the present invention is a novel, high-efficiency, and low-cost non-dissipative balancing technology based on one transformer, the following discussions are focused on several prior art methods, each of which performs non-dissipative balancing based on one transformer.
U.S. Pat. No. 8,598,844 (Densham et al.) discloses a method of balancing a plurality of battery cells, each of which is coupled to one of a plurality of secondary windings of a transformer during re-charging; however, the method cannot balance cells when the battery pack is discharging. U.S. Pat. No. 8,310,204 (Lee et al.) discloses a method of balancing one cell to the rest of a battery pack via a fly-back transformer; however, this method does not allow transferring charge from the pack to a cell, and does not allow transferring charge from a cell to other specific cell(s).
U.S. Pat. No. 7,400,114 (Anzawa et al.) discloses a method of balancing a battery string by utilizing a shared transformer with a plurality of pairs of primary and secondary windings corresponding to a plurality of battery cells; all the primary windings are switched on and off simultaneously then charge battery cell(s) with lower voltage(s) via secondary windings. However, the efficiency is low because every cell will be discharged then charged, even though the cell(s) with higher voltages will be discharged more and the cell(s) with lower voltage(s) will be charged more. And there will be considerable charge energy dissipated as heat via all the rectifier diodes, all the windings, and other components. And the method does not allow selection of transferring charge from some specific cell(s) to other specific cell(s).
U.S. Pat. No. 5,821,729 (Schmidt et al.) and U.S. Pat. No. 8,269,455 (Marten) disclose similar methods, each of which is for balancing a battery string by utilizing a shared transformer with a plurality of windings corresponding to a plurality of battery cells. Each winding can be driven bi-directionally via a full-bridge or a half-bridge configuration. And all the windings are energized simultaneously so that charge from cell(s) with higher voltages may be transferred to cell(s) with lower voltages in a forward-converter manner. These methods do not allow selection of transferring charge from some specific cell(s) to other specific cell(s). And voltage differentials among battery cells may be insignificant (for instance, the middle portions of discharge curves of some lithium-ion battery cells are very flat making it impractical to generate sufficient voltage differentials among battery cells), therefore making these methods impractical for most real world applications. And non-dissipative balancing that involves all the electrical energy units is inefficient because of various unnecessary energy losses resulting from charging and/or discharging multiple electrical energy units which are already approximately balanced.
The most common method of balancing series-connected super-capacitors (also known as ultra-capacitors) uses bleeding resistor(s) because of ease of implementation and low cost. U.S. Pat. No. 8,198,870 (Zuercher) discloses such a method; however, the method is essentially a passive/dissipative balancing method and cannot move extra charge to where it is needed.